Why Has My Electric Shower Switch Melted?

[barmy]

I have an electric shower (8KW) with a switch outside the bathroom. The shower failed to work on Wednesday so I checked the switch and the internal block has melted leaving a pungent 'fishy' smell though there was no apparent smoke emitting from it. Does any one have an idea why this has happened and if I replace it on Saturday things will be OK again? The switch was probably 10 years old and it looks like there was a short but not sure why it shorted if it did, any ideas folk?

[The Builder]

Either it's a very poor quality switch, or it's been overloaded.
What is the size of the breaker (or fuse) that protects the shower circuit?

Switch off at the board and examine the switch. Does it tell you its current rating?

It should be something like this.................

http://www.screwfix.com/p/crabtree-45a-dp-switch-neon/98632

[Old_Geezer]

It seems as if it passed a very large current and the circuit breaker failed to blow. But to get a large current there must be some short in the circuit in the first place. I'd be wary of the shower until you have identified the cause of both the current, and why the breaker failed to blow. Might be worthwhile paying an electrician to check it out.

[buildersmate]

What has happened is that the electrical resistance across the switch unit became too high, and the unit heated up. The heating led to surface oxidation which further increased the resistance and, bingo, the whole thing melts. The power across a resistance of just 0.2 ohms at 8 amps is 12Watts, more than enough to melt it in a tight space.
It could be the unit was a dodgy quality, or it could have happened because one of the terminal wiring connections was done screwed down adequately. Change the switch for another good quality one and pay special attention to the way you stripback and make the terminal connections.

[dekkypoo]

Quite common, what I've discovered in the past is when first fitted the terminals were not screwed tight enough so after a while the distance between the cable and the terminal gets slightly bigger and you end up with an 'arc welder' I've a collection of joint boxes, switches and sockets where this has happened. Probably no problem with the circuit or appliance, it just takes times for the gap to widen enough for it to get hot enough to melt the bits.